The concept of transmitting several television channels over a single-mode optical fiber via analog intensity modulation of a semiconductor laser diode has been receiving considerable attention. As proposed in the prior art, an arrangement would transmit multi-channel amplitude modulated-vestigial side-band (AM-VSB) signals, as used in present day cable television (CATV) systems, in an optical fiber transmission media. Such an arrangement would be useful in a CATV trunk system or in a fiber-to-the-home network. Optical fiber transmission systems that use the frequency division multiplexed AM-VSB format deliver a signal that is compatible with present day television and VCR equipment. They also have advantages such as simplicity of design, reduced bandwidth requirements for lightwave components, and much lower costs than digital or frequency-modulated alternatives.
The low loss of optical fibers make analog sub-carrier modulation an attractive technology. Several signals at different sub-carrier frequencies, each signal representing one of the television channels to be multiplexed, are summed and applied concurrently to the input of the laser device. The resulting laser drive current is a dc bias level plus the set of AM-VSB sub-carrier signals. For the laser, the magnitude of the optical output power from the laser is an approximately linear function of its drive current. The resulting sub-carrier frequency division multiplexed (FDM) output signal is applied to an optical fiber for transmission over an extended distance.
Multi-channel AM-VSB signal transmission requires special restrictions on the power, the linearity, and the intensity noise of the transmitting laser diode. For adequate system performance, the laser output light intensity must be a linear function of its drive current. Strict limitations on laser diode linearity are required because of the wide dynamic range of the National Television Systems Committee (NTSC) standard video format. Lasers with fairly linear characteristics are available with composite second and third harmonic distortion down to -30 dBc and -40 dBc, respectively, from the relevant carrier fundamental for acceptable levels of input modulation current. In that NTSC standard video format, the ratio of the magnitude of the total composite of the third order intermodulation distortion products at the carrier frequency to the magnitude of the carrier must be less than approximately -60 dBc. Similarly, the peak second-order distortion, i.e., the sum of several tens of two-tone products (or the ratio of the largest composite second-order peak to the carrier), must be less than approximately -50 dBc. This low distortion must be obtained when the laser is modulated with an optical modulation depth of typically one percent-ten percent per channel, to insure acceptable carrier-to-noise performance. To obtain such high signal quality in view of the large number of distortion products, the transmitting laser light power versus drive current characteristic curve must be extremely linear.
The two-tone third-order intermodulation distortion and the second harmonic distortion, respectively, must be near 96 dB and 70 dB lower than the video carrier, or better, for each channel.
Frequency division multiplexed signals generate intermodulation distortion because of any nonlinearity in the laser diode light versus current (L-I) characteristic. In frequency division multiplexed optical fiber systems, second-order laser nonlinearities create energy transfers from the applied sub-carrier frequencies to the sum and difference frequencies of all of the pairs of the applied signal frequencies. Third-order nonlinearities generate distortion products at frequencies corresponding to combinations of three applied signals. Such energy transfers cause undesirable intermodulation distortion and interference, both of which can limit the performance of the transmission system. There are several known causes of nonlinearity in semiconductor laser diodes. Some of the causes of nonlinearity are high frequency relaxation oscillations, low frequency heating effects, damping mechanisms, such as gain compression and nonlinear absorption, and leakage current. The resulting effect of the distortion is interference in the signal received further along in the system.
Although the most attractive architecture for multiplexing multiple video channels onto a continuous-wave laser output is by way of amplitude modulated-vestigial sideband signal multiplexing, the previously available semiconductor lasers exhibit distortions that fall short of meeting the strict linearity requirements for multi-channel frequency division multiplexing of television signals for CATV systems within the desired range of multiplex frequencies. Thus prior art multiplex systems cannot effectively perform the desired multiplexing operation. The problem is to develop a system with a sufficiently linear semiconductor laser diode so that the laser diode limits undesirable intermodulation distortion to an acceptable level when that diode is driven by a frequency division multiplexed group of television signal channels.